1. Field of the Invention
The present invention relates to a vehicle height control system, and more particularly, to a vehicle height control system comprising four actuators for controlling vertical positions of a pair of front wheels and a pair of rear wheels relative to a vehicle body to obtain a desired longitudinally and laterally balanced posture of the vehicle body, said four actuators being operated based upon output signals of three vehicle height sensors.
2. Description of the Prior Art
It is known in the art of modern automobile to incorporate such a vehicle height control system in the suspension structure of an automobile that comprises four actuators for controlling vertical positions of a pair of front wheels and a pair of rear wheels relative to a vehicle body so as to maintain a determinate longitudinally and laterally balanced posture of the vehicle body against unbalanced loading on the vehicle body. Such a vehicle height control system incorporates, when constructed according to an orthodox control concept, four vehicle height sensors for detecting the vertical positions relative to the vehicle body of the pairs of front and rear wheels so that each said actuator is operated based upon a difference of the vertical position relative to the vehicle body of each wheel from a determinate target value therefor so as to cancel said difference according to the conventional feedback control concept.
However, as a matter of principle it is possible to operate such four actuators for controlling the vertical positions of the pairs of front and rear wheels relative to the vehicle body so as to maintain a determinate longitudinally and laterally balanced posture of the vehicle body with three vehicle height sensors two of which are arranged as spaced along a longitudinal direction of the vehicle body while two of which are arranged as spaced along a lateral direction of the vehicle body so that, for example, the three vehicle height sensors detect the vertical positions of three of the pairs of front and rear wheels except a fourth one of the four wheels. In fact, it has been proposed in Japanese Patent Laying-open Publication No. 62-77209 to provide two such vehicle height sensors for a pair of front wheels and a third such vehicle height sensor for one rear wheel. In a prior art referred to in the above publication the actuators for the pair of front wheels are independently operated based upon the vertical position detecting operation of the corresponding pair of vehicle height sensors, while the actuators for the pair of rear wheels are synchronously operated based upon the vertical position detecting operation of the single vehicle height sensor applied to one of the pair of rear wheels. In the above-mentioned Japanese publication it is further proposed to suspend the vehicle height control operation of the actuators for the pair of front wheels when a comparison of output signals from the pair of vehicle height sensors for the pair of front wheels increases so much as to indicate that one of the pair of wheels has run on a rock or the like or has fallen in a hole or the like or the vehicle body has been jacked up at said one wheel.